Different Roles of Membrane Potentials in Electrotaxis and Chemotaxis of Dictyostelium Cells

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Abstract
Many types of cells migrate directionally in direct current (DC) electric fields (EFs), a phenomenon termed galvanotaxis or electrotaxis. The directional sensing mechanisms responsible for this response to EFs, however, remain unknown. Exposing cells to an EF causes changes in plasma membrane potentials ( V m ). Exploiting the ability of Dictyostelium cells to tolerate drastic V m changes, we investigated the role of V m in electrotaxis and, in parallel, in chemotaxis. We used three independent factors to control V m : extracellular pH, extracellular [K + ], and electroporation. Changes in V m were monitored with microelectrode recording techniques. Depolarized V m was observed under acidic (pH 5.0) and alkaline (pH 9.0) conditions as well as under higher extracellular [K + ] conditions. Electroporation permeabilized the cell membrane and significantly reduced the V m , which gradually recovered over 40 min. We then recorded the electrotactic behaviors of Dictyostelium cells with a defined V m using these three techniques. The directionality (directedness of electrotaxis) was quantified and compared to that of chemotaxis (chemotactic index). We found that a reduced V m significantly impaired electrotaxis without significantly affecting random motility or chemotaxis. We conclude that extracellular pH, [K + ], and electroporation all significantly affected electrotaxis, which appeared to be mediated by the changes in V m . The initial directional sensing mechanisms for electrotaxis therefore differ from those of chemotaxis and may be mediated by changes in resting V m .

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